The present invention relates to data storage devices and, in particular, to bit-patterned media and servo sector null patterns of bit-patterned media.
In a conventional magnetic disc drive, data is stored on one or more discs, which are coated with a thin magnetically hard layer. The magnetic layer itself is composed of a single sheet of very fine, single-domain grains. Every information bit of data is stored by many grains. This granular recording medium is typically divided into a plurality of generally parallel data tracks, which are arranged concentrically with one another perpendicular to the disc radius.
To guarantee good signal-to-noise ratio using such conventional magnetic media, the number of grains in every bit should be above a certain level. In order to save more bits on the given disc area (i.e., to increase the areal density), the size of a single grain is decreased. This is called scaling.
It is understood that scaling is limited by the onset of superparamagnetism: if the grain size is too small, the magnetic energy of grains is not sufficiently large compared to the thermal energy so that the magnetization becomes unstable and information can no longer be stored reliably. Superparamagnetism puts an areal density limit of 0.5 Tb/in2 for perpendicular recording.
This areal density limit could be exceeded through the use of bit-patterned media. In a patterned medium recording, ordered arrays of discrete magnetic elements are used to store data. The magnetic elements are separated from each other by a non-magnetic material. Each of the discrete magnetic elements or islands is a single grain and stores one information bit.
Unlike the conventional magnetic media, which requires the alignment of many adjoining magnetic particles that must each be magnetized into the same polarity (either positive or negative) to define a bit of data, the adjacent magnetic elements or islands of the bit-patterned media can have the same or different magnetic polarities due to their separation from each other by the non-magnetic material. This difference should allow bit-patterned media to have a much higher areal density than the conventional magnetic recording media.
High areal density recordings using bit-patterned media will require the precise placement of the head relative to tracks of the media. Unfortunately, conventional null patterns used by servo systems for controlling the head position relative to tracks of the granular magnetic medium are incompatible with bit-patterned media.
Aspects of the present embodiments provide solutions to these and other problems, and offer other advantages over the prior art.